Squelch circuit for suppressing noise components



Feb. 1,- 1949. R, E, H K 2,460,786

SQUELCH CIRCUIT FOR SUPPRESS'ING NOISE COMPONENTS Filed June 4, 1946 ATTORNEY Patented Feb. 1, 1949 SQUELCH CIRCUIT FOR SUPPRESSING NOISE COMPONENTS Robert E. Schock, ltiverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 4, 1946, Serial No. 674,181

3 Claims.

In telegraph signal systems when the signals stop, noise in the system often excites the tube stages, thereby operating the recording apparatus to cause the same to print gibberish.

The primary object of my invention is an improved signal relaying and amplifying system for the signals to be recorded including improved means for inhibiting or squelching operation of the tube stages in the absence of signals to thereby prevent improper operation of the recording apparatus.

In describing my invention, reference will be made to the drawings wherein the single figure illustrates signal amplifying and recording apparatus control circuits arranged in. accordance with my invention.

To simplify the description of my invention, it has been applied to a recorder of signals, such as are derived at the output of a frequency shift receiver as disclosed in Schock et a1. application Serial Number 632,978, filed December 5, 1945. The signals at the discriminator and detector output in this receiver swing above and below and about zero potential. It will be understood that theinvention is equally applicable to on-off telegraph systems of the type wherein the signal is varied in a like manner and to F. S. and on-off systems of the type wherein the signal is varied from some high negative value representing, say, mark, to about zero value representing, say, space.

In the system being described, the numerals l to 22 inclusive designate the essential circuit elements of a tone keyer as used by my assignee,

the Radio Corporation of America, except for amplified in A and filtered in LF and applied detector, the resistor 5 is inserted. When current flows in the grid circuit of tube 4 the potential drop in 5 tends to limit such current flow. The tube 4 is fully conductive when zero bias is applied to its grid and the positive swings of the applied potential, which may represent space characters, do not change this condition,but the negative swings, which may represent mark characters, bias the tube 4 to cut-off. When the tube 4 is conductive the plate current drop through resistor 6 is fed to the grids of the tone keyer stage tubes 8 and I0 through the secondary windings of the transformers 20 coupled Withthe tone source 3. This potential drop which makes the grids of tubes 8 and I0 less positive drives tubes 8 and ill to cut-off so that the tone applied from source 3 to the grids of the. amplifiers 8 and H1 through transformer 20 is not passed to the tone keyer output transformer 22 and thence to the lines 23 which may go to recording apparatus such as a teletypewriter. When the potential at the grid of tube 4 swings negative in, say, mark condition, the tube A is cut-oil so that its plate current falls and the potential at the tube 4 anode rises. This swing inthe positivedirection of the biasing potential on the grids of tubes 8 and I0 causes the same to become conductive to amplify the tone from source 3 and feed it to the output transformer 22 and thence to the recording apparatus connected with the lines 23.

, Potentials for the anodes and screen grids of tubes 4, 8 and H) are supplied from appropriate points on a potentiometer including resistors l2, l4, l6 and I8 shunting a direct current source. The tone keyer including tubes 4, 8 and 10 may be substantially as disclosed in Peterson Patent 2,237,631 dated April 8, 1941.

The purpose of theinvention is to block ofi tubes 8 and I0 when signals no longer appear acrossresistor 2, and my improved means to do this will now be described. The signal potentials at 2 are also applied across the potentiometer resistor 24 a point on which is coupled to the grid of an amplifier tube 26 which in turn feeds the primary winding of a transformer 28 the secondary winding of which is coupled in push pull by diode rectifiers 30 and 32. The rectifier circuits are completed by resistor 34 connecting the anodes of the diodes 3B and 32 to a point intermediate the terminals of the transformer 28 secondary winding. The rectified output of rectifiers 30 and 32 appears as a negative voltage across the rectifier load resistor 34 and condenser 36. The negative voltage appearing across the resistor 34' is applied through a grid limiting resistor 38 to the control grid of the tube 4 whose plate circuit is connected in parallel with the plate circuit of the control tube 4 in the tone keyer. The tube 40 is herein designated as a squelch tube and it derives its plate current through the load impedance 6 just as does tube A negative bias is'suppliedto the cathode of the squelch tube 4!! through'potentiometer 44.

In operation, potentiometer 25 is adjusted so that when a signal is present the magnitude of the negative voltage developed across the rectifier load resistor 34 will be such as to cut-ofi squelch tube 40. When the squelch tube 40 is cut-off, that is when signals are present, only the tube 4 is drawing current through the load resistor 6 and it operates in its normal manner as described above to key the output of tone source 3 in accordance with the signals to supply keyedtone output at the lines 23. Whenthere is no signal present, the full wave rectifier output approaches zero and the negative potential across the condenser 36 leaks off through resistor '34-so that the grid of tube 49 is no longer biased to cut off and tube '19 becomes conductive. Now since the plate of tube 60' is in parallel with the plate of tube 4 through resistor 6 ofthe tone keyer the current of tube/40 fiowing in resistor 6-supplies a voltage drop which cuts ofi tubes 8 and i tostop or block output from the tone source 3. When squelch tube 40 is conductive the tone keyer is rendered inoperative and when squelchtube 40 is out off the tone keyer is free to operate normally. The time constant of the network including resistors 34 and 3.6 is asdescribed above and is large enough to insure that discharge into the region where the tube 40 is turned on does not take place in normal signaling. The time constant is, however, small enough so that it will discharge into that region after the loss of a number of signal elements, for example, those of a letter word. Since the tube 40 becomes conductive only in the absence of signal its action on the tone keyer is that of a squelch circuit.

' 1 Of course noise on the circuit input, in the absence of signal, will be rectified and appear as a negative voltage'on thegrid of the squelch tube thus cutting it off and allowing the tone keyer to become operative. This would defeat the purpose of the squelch since the noise would then be able to operate the tone keyer in the absence of signal.

, 'I o obviate this possibility, a negative voltage is supplied to the cathode of tube 40 through potentiometer ill. (This is, of course, equivalent to-applying a positive bias on the grid. In fact if the cathode of tube 40 were grounded and a positive bias placed in series with the grid circuit it would accomplish the same end.) With a negative potential on the cathode (positive grid bias) the noise must now be of much greater amplitude before it can generate enough negative potential in the rectifier output to overcome the positive bias on the squelch tube grid. Therefore it is possible to adjust the bias applied by potentiometer 44 so that noise cannot render the squelch inoperative unless its amplitude is almost as great as that attained by the signal when the signal is 'present. As an example, suppose the signal when presentgenerates volts negative potenr tial cross'resistor 34. Since tube 40 would normally best be a sharp cutoif tube which would cut off at about 1 volt bias, only a volt or two of this signal is needed for cutting the tube 40 off. If the cathode of tube 40 is made 8 volts negative it will require 9 volts of rectified signal to shut it off. Since 10 volts of rectified signal is available, when the signal is on, the signal will out 01f the squelch with a margin of one volt. In the absence of signal, however, the noise now has to have an amplitude which will give 9 volts of rectified .output across resistor 34 before it can stop the squelch'ac'tion. Therefore the tone keyer remains squelched in the event of no signal even though the noise is eight tenths as strong as the signal used. That is, the noise will have to. generate more than 8 volts across resistor 34 before it begins to cut tube 40 off and actually will have to generaitte 9 volts before it can completely cut tube 40 t The potentiometer 24 is used to adjust the level of the signal in the squelch circuit to a magnitude which will insure that the squelch tube 40 is cut oif in the presence of signal. The tube 26, rectifiers 3D, 32, load 34 and 36, etc. are such as to generate at the grid of tube 40 a negative voltage from the incoming signal sufficient to cut off the Since the tubes and 32 form a full wave rectifier circuit both sides of the signal wave get rectified and the same voltage is generated from the signal regardless of whether the keying is light or heavy. The potentiometer 44 sets the threshold below'which noise will be unable to operate the squelch.

'What is claimed is:

1. In 'a system for relaying telegraphy signals and for preventing relaying of noise components in the absence of signals; a first control tube having input electrodes excited by said signals and noise and having output electrodes in circuit with an impedor wherein a potential is developed which may be used, a rectifier excitedby said signals, said rectifier having an-output impedor wherein 'a ,potential is developed which is negative in the presence of signals, a. second tube having a controlgrid coupled to said rectifier and excited by said last namedpoten'tial and having an anode circuit including the impedor in the output circuit of said firsttube, and means for-biasing said second tube by a potential such that the same is ut oif in the presence of signals at the input of said rectifier.

2. In a system for relaying telegraphy signals and for preventing relaying of noise components in the absence of signals, a first control tube having input electrodes excited by said signals and noise and having output electrodes in circuit with an impedor wherein a potential is developed which may be used, a rectifier excited by said signals, said rectifier having an output impedor wherein a potentialis developed which is negative in the presence of signals and less negative in the absence of signals, a, relatively large capacitor in shunt to said impedor, a second tube having a control grid coupled to said last named impedor and excited by said last named potential and having an anode circuit including the first named impedor inthe output circuit of said'first tube, and means for biasing said second tube by a potential such thatthe same is cut off in the presence of signals andis conductive in the presence of noise components which are materially less in magnitude than the signals.

3. In a telegraph system a tone source and apparatus for keying said tone source in accordance .with signals represented by potentials of varying magnitude and for preventing keying of said tone source by noise components in the absence of signals comprising, a first control tube excited by said signals and noise, an output resistor wherein a potential is developed which keys said tone source, a full wave rectifier circuit excited by said signals and noise, said full wave rectifier circuit including an output impedor wherein a potential is developed which is negative in the presence of signals and less negative in the absence of signals, a relatively large capacitor in shunt to said impedor, a second control tube having a control grid coupled to said impedor and excited by said potential, and having an anode circuit including the resistor in the output circuit of said first control tube, and means for bias- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,034,970 Bond Mar. 24, 1936 2,237,631

' Peterson Apr. 8, 1941 

